1. Technical Field
The present invention relates to photovoltaic devices, and more particularly to heterojunction photovoltaic devices fabricated using a low temperature process.
2. Description of the Related Art
Ge solar cells are used as the bottom cells in high-efficiency multi junction solar cells. Since Ge is an expensive material, it is desired to fabricate solar cells on thin layers of Ge transferred from a boule or wafer, onto a handle substrate. The electrical junctions in conventional crystalline Ge solar cells are formed by high temperature processes such as diffusion, which are not compatible with typical low-cost handle substrates such as plastic. Therefore, low temperature processes are highly desired for post-processing of thin Ge wafers transferred onto low-cost handle substrates. In addition, lowering the process temperature may reduce the fabrication cost of the solar cell regardless of the usage of a handle substrate, as well as allowing the usage of low cost Ge wafers which may be degraded at high process temperatures.
Referring to FIG. 1, the structure of the most high-efficiency stand-alone solar cells includes an n+ emitter c-Ge contact 10 formed by phosphorous diffusion from a spin-on-dopant at ˜600° C. and a p+ c-Ge back-surface-field contact 12 is formed by screen printing or deposition of Al, followed by annealing at temperatures above the Ge—Al alloy eutectic temperature (˜425° C.). The emitter contact 10, a substrate layer 14 and the back-surface-field contact 12 all include crystalline Ge (c-Ge). An emitter passivation layer 16 may be provided by a plasma enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous Si (a-Si:H). The passivation layer 16 improves the solar cell efficiency by reducing the recombination of electron-hole pairs at the surface of the emitter layer (n+ c-Ge). However, the passivation layer 16 is not fundamental to the device operation and may be omitted. The front (emitter) contact 10 includes the passivation layer 16 with metal fingers 20 formed through the layer 16 by lithography or by diffusion of metal through this passivation layer 16 at about 200° C. The back contact 12 includes an Al layer 18. The best open-circuit voltage achieved for these cells is ˜270 mV. The low open circuit voltage is due to (i) the low bandgap of Ge, and (ii) the lack of sufficient surface passivation at the front and back of the cell.